As mentioned earlier, low-current drop out is the normal way in which the SCR is turned off. Gate-controlled switch is designed for easy opening with a reverse-biased trigger. A gate controlled switch (GCS) is closed by a positive trigger and opened by a negative trigger (or by low-current drop out). Gate-controlled switch circuit is shown in figure. Each positive trigger closes the GCS, and each negative trigger opens it. Thus a square-wave output is obtained, as shown in the figure.
The most obvious advantage of GCS over the SCR or SCS is the fact that it can be turned on or off by applying the proper pulse to the cathode gate (without the anode gate and associated circuitry required for the SCS). A second very important advantage of GCS is improved switching characteristics.
The gate-controlled switch is useful in counters, digital circuits, multivibrators, voltage regulators and other applications in which a negative trigger is available for turn-off.